Increasing demands for personal mobile communications equipment have motivated recent research activities to focus on the development of inexpensive, small size, low power consumption, and low noise level systems. To satisfy these requirements, one of the most important and indispensable circuit components is the on-chip silicon-based inductor.
As a result, miniaturization of the inductor on silicon has become a current key research area and extensive work has been done in this area. However, despite efforts by many researchers having skill in the art, achieving high performance on-chip inductors, i.e., high qualify factor (Q), still remains a major problem especially when radio frequency integrated circuits (RFICs) are built on silicon.
Conventional inductors built on silicon are generally planar in nature. The current complementary metal oxide semiconductor (CMOS) process uses a very conductive substrate. Spiral inductors fabricated on such a lossy substrate suffer from high capacitive and magnetic losses.
In addition, high dynamic resistance of metal lines at GHz frequency ranges further degrades the inductor performance in CMOS technology as compared to those fabricated in monolithic microwave integrated circuits (MMICs).
Many fabricating techniques, processes, and materials have been proposed to improve the performance of on-chip inductors. Tedious processing techniques such as etching away the silicon substrate under the inductor have been introduced to remove the substrate parasitic effects completely. Despite achieving good results, industries are reluctant to adopt such a technique because of reliability issues such as packaging yield, as well as long-term mechanical stability.
Another approach to minimize the substrate loss for silicon-based inductors has been to increase the substrate resistivity. This technique has yielded significant results, however, the substrate becomes unsuitable for building active MOS devices.
The most critical factor hindering the performance of silicon-based inductors is the high resistive aluminum-copper (AlCu) interconnects used in silicon processes.
In comparison, thicker and less resistive gold (Au) metalization together with lossless substrate in gallium arsenide (GaAs) technology permits high performance inductors to be fabricated easily. To overcome high metalization resistance, a popular technique is to have the layers of metal stacked together, thereby achieving a high Q inductor.
Another possible alternative is to use an active inductor. In an active inductor high Q factor and inductance can be achieved in a really small silicon area. However, such approach suffers from high power consumption and high noise levels that are not acceptable for low power and high frequency applications. In addition, performance of active inductors are very sensitive and dependent upon the inductor's biasing circuitry, making it time consuming and tedious to design.
As a result of the above, the simplest and most commonly used on-chip inductors are planar silicon-based spiral inductors, which require careful layout optimization techniques to improve performance.
In the conventional spiral inductor design, the inductor is planar and fabricated on a conductive silicon substrate. To improve the Q factor of the spiral inductors, the top metal is usually stacked with a few layers of lower metal through vias to minimize the overall metal series resistance. Nevertheless, when more layers are used to realize a very thick conductor, the whole spiral is brought closer to the substrate. This increases the spiral-to-substrate parasitic capacitance and hence results in a degradation of Q factor as well as the inductor's self-resonant frequency. It has been observed that the Q factor of a 4-layer stacked inductor decreases at a much faster rate compared to 1- to 3-layer stacked inductors. Because of this, it becomes extremely difficult to design high performance inductors with large inductance values since such a phenomenon is more pronounced when the inductors occupy large areas.
Magnetic losses occur when inductors are built on conductive substrates. According to Faraday's law, an image current or eddy current is induced in the substrate underneath the spiral inductor. Since a silicon substrate has low resistivity, this image current can flow easily. In compliance with Lenz's law, the direction of flow for this induced current is opposite to that of the inductor current. This characteristic results in a degradation of the inductor's overall useful inductance.
To reduce these magnetic losses due to the formation of the eddy currents, varying width inductors have been proposed. The basic working principal is to minimize undesirable magnetic flux created by the induced substrate eddy current, especially at the inductor's core. This can be easily accomplished by reducing current density of the inductor's inner turns. Meanwhile, when current density of the inner turns is reduced, induced substrate current at the center of the inductor will also have a lower current density. As a result, at the inductor's core, parasitic magnetic flux generated in the substrate is much weaker and this helps increase the inductance and the Q factor of spiral inductors.
However, it has been observed that the Q factor degrades drastically at higher frequencies when compared with fixed-width spiral inductors. This suggests that for a large inductor to achieve a difference in current density between the inner and outer turns, the overall inductor area must be enormous. Its Q factor is expected to fall even before 2.45 GHz and would, of course, render this technique completely useless.
Solutions to these problems have been long sought, but have long eluded those skilled in the art.